get_text: fail on unrecognized option instead of silently defaulting

Per review feedback from julian-smith-artifex-com: the intent was always
to raise on an unrecognized option, not fall back to "text" silently.
Removes the fallback and keeps the assert (which the earlier fix in this
PR had already de-duplicated from its own missing-comma bug elsewhere).

Author: A831ARD0

src/utils.py

@@ -491,8 +491,7 @@ def get_text(
         "blocks": pymupdf.TEXTFLAGS_BLOCKS,
     }
     option = option.lower()
-    if option not in formats:
-        option = "text"
+    assert option in formats
     if flags is None:
         flags = formats[option]